The present invention relates to inspecting metal tubes by eddy currents, and a major, although not exclusive, application thereof lies in inspecting the cladding tubes of rods containing a neutron-absorbing material or a fertile material for use in the control clusters of a nuclear reactor.
A method of inspecting the rods of control clusters by eddy currents that is already known (FR-A-2 585 869) corresponding to U.S. Pat. No. 4,741,878 to Gebelin et al makes use of coils through which the rods are caused to pass. The coils are powered by current at high frequency. Analyzing the voltage across the terminals of the coils and representing the impedance thereof makes it possible to evaluate tube wear.
That solution gives results which are satisfactory so long as the tubes are homogeneous, e.g. being made of stainless steel. However, it is no longer reliable if it uses conventional coils and the tubes have respective outer surface layers whose magnetic or electrical characteristics are significantly different from those of the bulk of the metal, e.g. because the tubes have been subjected to surface treatment for the purpose of reducing wear.
This applies in particular when the tubes are made of steel and have a surface layer that is 10 .mu.m thick to several tens of .mu.m thick, either of chromium which has electrical conductivity that is about six times greater than that of the base metal, or that is enriched in nitrogen obtained by electrical discharge in a rarified atmosphere containing nitrogen (ionic nitriding) and which is ferromagnetic.